Applicant claims priority in and hereby incorporates by reference in its entirety Japanese Application No. 2001-194935 (P), filed Jun. 27, 2001.
The present invention relates to a transistor element that is formed in a SOI (Silicon On Insulator) film, and more particularly includes a semiconductor device that improves a current drivability of a partially depleted type element in which its body is partially depleted.
A SOI MOSFET is a MOSFET structured in a silicon single crystal formed on an insulation film, and has an advantage in that its source-to-drain junction capacity is suppressed to a small amount. For this reason, it operates faster than a MOSFET manufactured on an ordinary bulk silicon substrate (i.e., a bulk MOSFT). Also, since it can operate at a high speed even with a low voltage power supply, its application to lower power consumption LSIs is being studied.
SOI MOSFETs are generally divided into two groups, fully depleted type SOI MOSFETs in which a body region in the silicon single crystal is fully depleted, and partially depleted type SOI MOSFETs in which a body region in the silicon single crystal is partially depleted. Embodiments of the present invention relate to partially depleted type SOI MOSFETs, and to technologies that improve their current drivability.
FIG. 4 shows a cross section of a structure of an ordinary SOI MOSFET. A silicon single crystal 100 is formed on a SiO2 film that is called an embedded oxide film 102, and a P-type body 104 and N-type source and drain regions 106, for example, are formed. A gate electrode 108 is formed over the body 104 through a gate oxide film 110. Sidewalls (spacers) 112, which are to be provided after N-type extension regions 114 having a lower concentration than that of the source and drain regions 106 are formed, are formed on both sides of the gate electrode 112. Channel section 116 is located below the gate electrode 108.
In the partially depleted type SOI MOSFET with an FB (Floating-Body) operation, the body is in an electrically floating state. By this, its current characteristic during a circuit operation is different from that in a steady state because of the following two reasons:
The first reason is a self-heat generation effect resulting from the low thermal conductivity of the embedded oxide film. In general, the self-heat generation effect during a circuit operation is weaker than the effect in a steady state, and therefore the actual current drivability during a circuit operation is higher than that estimated from DC measurement results. It is noted that this effect is universal to SOI devices, and can be seen in operation methods other than the FB (Floating-Body) operation method (for example, in a BTS (Body-Tied-to-Source) operation to be described below).
The second reason is a so-called substrate floating effect. While a body potential in a steady state is determined by a condition in which the sum of currents flowing out from the body becomes zero (0), a body potential in a circuit operation needs to be calculated in consideration of a transient capacitive coupling between each node of the source and drain and the body.
If a normal value of off current is considered, a body potential in a steady state at an off bias (a gate voltage Vgs=0, and a drain voltage Vds=VDD (power supply voltage)) becomes an issue. Also, when an effective current characteristic during a circuit operation is considered, a body potential in a transient state at that moment becomes an issue. In this connection, a parasitic bipolar operation caused by generation of carriers in a steady state, which is characteristic to SOI MOSFETs with an FB (Floating-Body) operation, will be described below.
In an ordinary bulk MOSFET, a substrate potential is fixed at a power supply line potential through a well contact. A substrate current that penetrates the well contact is a current caused by the generation of carriers (impact ionization current may have a contribution depending on the setting of the power supply voltage and off current) at a junction thereof, which is caused by a potential difference applied between the drain and the substrate.
However, a SOI MOSFET with an FB (Floating-Body) operation, a terminal that corresponds to a substrate is not provided in the body. As a result, a current by generated carriers at a junction section of the drain and the body positively bias the body.
In other words, the current by generated carriers at a junction section of the drain and the body plays a role of a base current, such that a bipolar operation parasitically occurs with the source, the drain and the body functioning as an emitter, a collector and a base, respectively.
The body is positively biased by the parasitic bipolar operation, and carrier recombination currents at the source-body junction section increase. Eventually, the body potential continues elevating until it balances with the current by generated carriers at the drain-body junction section. Therefore, a stationary value of the body potential is given by the following expression:
Ibs(Vbs)+Ibd(Vbd)=Idiode(Vbs)+Idiode(Vbsxe2x88x92Vds)=0xe2x80x83xe2x80x83Equation (1)
(where Ibs is a body-to-source current, Vbs is a body-to-source voltage, Ibd is a body-to-drain current, Vbd is a body-to-drain voltage, Idiode is a diode current including impact ionization current, and Vds is a drain-to-source current.)
By the above SOI MOSFET with an FB (Floating-Body) operation, an off current during a circuit operation standby period increases in association with the parasitic bipolar operation resulting from currents by generated carriers. As a result, the SOI MOSFET needs to increase its impurity concentration at the body (channel section) compared to a bulk MOSFET, and its carrier field effect mobility decreases accordingly.
Certain embodiments of the present invention relate to a semiconductor device having a SOI MOSFET in which a MOSFET is formed in a silicon single crystal formed on a dielectric film. The semiconductor device may include a SOI MOSFET with a BTS (Body-Tied-to-Source) operation accompanied by a transient capacitive coupling of a body during a circuit operation, which is achieved by providing a body terminal in which a resistance between itself and the body in the silicon single crystal is positively increased, and connecting the body terminal to a source terminal.
Certain embodiments also relate to a semiconductor device having a SOI MOSFET in which a MOSFET is formed in a silicon single crystal formed on a dielectric film, the semiconductor device including a SOI MOSFET with a BTS (Body-Tied-to-Source) operation that satisfies three conditions as follows:
Condition 1, wherein a capacitive coupling occurs during a circuit operation:
Rb greater than Rrc,
wherein Rb is a body-to-body terminal resistance (body resistance), and Rrc is a resistance dependent on a circuit operation frequency.
Condition 2, wherein A body potential under a steady state becomes about zero at an off bias (in which a gate voltage Vgs=0, and a drain voltage Vds=VDD (power supply voltage)):
Vbs,standby=xe2x88x92[Idiode (Vbs,standby)+Idiode (Vbs,standbyxe2x88x92VDD)]xc3x97Rb≅0,
wherein Vbs,standby is a body potential in a steady state at an off bias, Idiode is a diode current including impact ionization current, VDD is a power supply voltage, and Rb is a body-to-body terminal resistance (body resistance).
Condition 3, wherein a current that penetrates a body terminal during a circuit operation is sufficiently larger than a current that flows upon generation and recombination of carriers:
|Ib| greater than  greater than |Idiode|,
{wherein Ib is a penetration current at the body terminal, and Idiode is a diode current including impact ionization current.
Certain embodiments also relate to a semiconductor device having a SOI MOSFET in which a MOSFET is formed in a silicon single crystal formed on a dielectric film, the semiconductor device including a body lead-out section in which an impurity is introduced for leading out a terminal of a body in the silicon single crystal, a resistance factor section using a depletion phenomenon adjacent to a boundary between the impurity of the body lead-out section and the body, and a SOI MOSFET with a BTS (Body-Tied-to-Source) operation that satisfies three conditions as follows:
Condition 1, wherein a capacitive coupling occurs during a circuit operation:
Rb greater than Rrc,
wherein Rb is a body-to-body terminal resistance (body resistance), and Rrc is a resistance dependent on a circuit operation frequency.
Condition 2, wherein a body potential under a steady state becomes to be about zero at an off bias (in which a gate voltage Vgs=0, and a drain voltage Vds=VDD (power supply voltage)):
Vbs,standby=xe2x88x92[Idiode (Vbs,standby)+Idiode (Vbs,standbyxe2x88x92VDD)]xc3x97Rb≅0,
wherein Vbs,standby is a body potential in a steady state at an off bias, Idiode is a diode current including impact ionization current, VDD is a power supply voltage, and Rb is a body-to-body terminal resistance (body resistance).
Condition 3, wherein a current that penetrates a body terminal during a circuit operation is sufficiently larger than a current that flows upon generation and recombination of carriers:
|Ib| greater than  greater than |Idiode|,
wherein Ib is a penetration current at the body terminal, and Idiode is a diode current including impact ionization current.